The need for classification separation of small particles from mixtures of two or more different types or sizes of such particles arises in many different industrial fields. For example, in minerals extraction processes, mineral ores are crushed and milled, and particles of the valued component thereof need to be concentrated and separated from the less valuable rock (the gangue). Powders are frequently used as raw materials for pigments, metals, molding sands, ceramics, foods, drugs and the like. To ensure the quality of these products, it is often necessary to analyse and control the particle size and particle size distribution of the powders. Methods and equipment by use of which particle sizes of powders could be determined, and mixtures of particles could be sorted according to size, in a rapid and efficient manner, would find a wide variety of applications in industry, both as laboratory instruments and processing aids.